1. Field of the Invention
This invention relates to an image forming apparatus which develops the electrostatic latent image formed on a photo-sensitive drum, with toner having a small particle size, and transfers the image onto a paper sheet by an image-transferring roller or belt.
2. Description of the Related Art
In recent years, the quality of an image formed by electrophotography, one of image forming methods for copying machines or printers, has been enhanced by, for example, using toner having a smaller particle size. Although the toner of a small particle size may easily scatter, is expensive, and requires high cleaning cost, its use has become an important technique for enhancing the quality of the image, partly because of an improvement in a technique of manufacturing finer toner particles (e.g. suspension polymerization).
In general, the toner image formed on a photo-sensitive drum is transferred onto a paper sheet by using corona transfer method, that is, by applying corona ions to the reverse side of the paper sheet.
More specifically, the periphery of the photo-sensitive drum is electrostatically charged by a charger, and then an electrostatic latent image is formed on the drum by exposure means. The latent image is developed into a toner image by a developing unit, and the toner image formed on the drum is transferred to a transferring region by rotating the drum. Sheets of paper are supplied from a paper cassette, one by one, in synchronism with the charging, exposing, and developing processes, and transferred to the transferring region by an aligning roller. In the region, corona ions are applied to the reverse side of the paper sheet by a corona charger, whereby the toner image is transferred onto the sheet. This sheet is then separated from the drum by an AC separating charger, moved to a fuser where the toner image is fixed, and discharged. The residual toner is removed from the drum by a cleaning device. Further, all static electricity on the drum is eliminated by a discharger, which is the termination of one cycle of copying procedure.
The present inventors measured the relationship between gradation and the particle diameter of toners.
FIG. 7 shows OD - ID curves indicating the relationship between the original density and image density, which were obtained when toners having average particle diameters of 5 .mu.m, 7 .mu.m, 9 .mu.m, and 11 .mu.m were used, respectively. The curve, obtained when toner having an average particle diameter of 10 .mu.m was used, is not shown, but was almost the same as that obtained when the toner of the average particle diameter of 11 .mu.m was used. As can be understood from the curves, the smaller the toner particle is, the more faithfully the original can be copied. The image was improved in gradation when the toner of the average particle diameter of 9 .mu.m or less was used.
Then, the inventors measured the resolution of the image developed on the photo-sensitive drum before transferred onto a paper sheet, and also the image transferred onto the sheet before being fixed by a fuser. The resolutions of the images, obtained when toners of various particle diameters were used, were measured against a predetermined resolution chart, by the use of a magnifying lens. The measurement results are shown in table 1.
Further, the transfer efficiencies of those portions of the respective images which had a density in the vicinity of 0.8 were measured. The measurement results are also shown in table 1.
As is shown in table 1, the smaller the toner particles, the higher the resolution of the image developed on the drum.
However, the smaller the toner particles, the lower the resolution of the image transferred onto the sheet. This is because the smaller the toner particles, the more greatly the toner scatters, and further, the lower the transfer efficiency. The fact that the transfer efficiency lowers as the toner particle diameter is small will be most clearly understood by comparing the transfer efficiencies of a solid portion of an image, obtained by using toners of various particle diameters.
As is described above, although the quality of the developed image can be enhanced by using toner of a small particle diameter, the small particle toner causes the image, transferred by the corona transfer method, to be greatly degraded.